1. Field of the Invention
The present invention relates to the structure of a joint in a frame such as a vehicle frame.
2. Description of the Related Art
The rear frames of some vehicles such as motorcycles are constructed of pipes of a light metal such as aluminum alloy or the like. The pipes are usually in the form of lightweight hollow pipes having a rectangular cross-sectional shape which imparts desired flexural stiffness to the pipes. In a joint between frame pipes, a solid spacer having a cross-sectional shape that is substantially the same as the cross-sectional shape of the frame pipes is fitted in the frame pipes across the joint for reinforcing joint regions that are fastened by a bolt or the like.
The frame pipe and the spacer should be held in intimate contact with each other to enable the bolt to fasten them firmly together. Various methods are known for keeping the frame pipe and the spacer in close contact with each other. According to one method, the spacer is machined to desired accurate dimensions that match the inside diameter of the frame pipe, and press-fitted into the frame pipe with small forces. Another process uses a stamping press to crimp the frame pipe after the spacer has been inserted into the frame pipe. According to still another practice, a slit is cut in the pipe joint, the spacer is inserted into the pipe joint through the slit, the pipe is pressed to bring the pipe into intimate contact with the spacer, and thereafter the slit is welded.
However, the above known methods are disadvantageous for the following reasons: The first method of machining the spacer to accurate dimensions matching the inside diameter of the pipe is tedious and time-consuming, and highly costly. The second process of crimping the frame pipe with the stamping press fails to bring the frame pipe neatly into intimate contact with the spacer because the frame pipe of rectangular cross section is highly rigid at the walls as well as the corners. Furthermore, the second process tends to develop cracks in the frame pipe when the frame pipe is pressed under increased forces. The third practice of forming the slit in the pipe joint is effective to prevent the pipe frame cracking, but results in a substantial expenditure of labor, time, and expenses as it needs slitting and welding processes.
It is therefore an object of the present invention to provide a frame joint structure which has a pipe neatly crimped on a solid spacer fitted therein without cracking.
According to the present invention, there is provided a frame joint structure comprising a joint comprising a frame pipe of metal having a hollow cross-sectional shape, and a solid spacer of metal having a cross-sectional shape which is substantially identical to the hollow cross-sectional shape of the frame pipe, and fitted in the frame pipe, the frame pipe being pressed and heated so as to be crimped on the solid spacer fitted therein.
When the frame pipe is pressed while being heated, the frame pipe is subjected to increased plastic deformation, and hence can be brought into intimate contact with the solid spacer without cracking and with minimized springing back.
The frame pipe is pressed in a direction along which the frame pipe arid the solid spacer will be fastened together by a fastener such as a bolt or the like. The frame pipe may be of a square cross-sectional shape, a rectangular cross-sectional shape, a hexagonal cross-sectional shape, an octagonal cross-sectional shape, or any of other polygonal cross-sectional shapes. The frame pipe may be made of a ferrous metal or a nonferrous metal.
The frame pipe may be pressed and heated by flat electrodes of a resistance welding machine. When flat side walls of the frame pipe are pressed by the flat electrodes while an electric current is flowing therethrough, the frame pipe can be pressed uniformly, providing flat seats for the fastener on the flat side walls thereof. The flat electrodes may have a width greater than the width of the frame pipe for uniformly pressing the frame pipe.
The frame pipe should preferably be made of an aluminum alloy. Pipes of an aluminum alloy having a rectangular cross-sectional shape, which is strong enough to be used as frames, tend to crack when pressed by a stamping press. However, when such pipes are pressed while being heated, they are prevented from cracking under pressure. Because frames that need to be lightweight are increasingly made of an aluminum alloy in recent years, the frame joint structure according to the present invention offers increased practical values.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.